1. Field of the Invention
The present invention relates to an exhaust gas filtration device for removing solidification constituents and solids in exhaust gas, which is provided in the exhaust path of a gas treatment chamber employed in a step of manufacturing semiconductor elements or electronic components.
2. Description of Related Art
In, for example, plasma CVD equipment which is used in the manufacture of semiconductor elements or electronic components, a plasma CVD process occurs in an airtight vessel, and an a-Si film or SiN film, etc. is deposited on a substrate. In this process, apart from on the substrate, the thin film is deposited on the inside wall etc of the airtight vessel. Usually, the thin film that is deposited on the inside wall etc is removed by plasma cleaning using NF3 gas. In this process, gaseous products chiefly represented by Si2F6(NH4)3.F* are produced in the airtight vessel. Evacuation of the airtight vessel is continued during plasma cleaning, so powder-form solids chiefly represented by Si2F6(NH4)3.F* are precipitated and deposited on the piping, etc. constituting the exhaust path. Such deposition of solids tends to cause blockage of the piping. In order to prevent this, an exhaust gas filtration device is provided in the exhaust path with the object of removing solidification constituents (gaseous products whose condition is changed to a solid by cooling or by densification (raised pressure)) and solids in the exhaust gas.
FIG. 13 is a block diagram illustrating a conventional exhaust gas filtration device. FIG. 13(A) shows an airtight vessel 10 and exhaust path 12a of this airtight vessel. A vacuum pump 14 and exhaust gas filtration device 16 are arranged in this exhaust path 12a. This conventional exhaust gas filtration device 16 comprises a trap device 18 and filter 20. The exhaust path 12a referred to above is constituted by connecting this vacuum pump 14, trap device 18 and filter 20 in this order from airtight vessel 10 by means of piping in accordance with requirements.
Also, the exhaust path 12b shown in FIG. 13(B) consists of trap device 18, filter 20 and vacuum pump 14 connected in this order through piping, as required, from airtight vessel 10. In this way, for the order of arrangement of the exhaust gas filtration device 16 and vacuum pump 14 an order may be employed that is the opposite of that of FIG. 13(A).
As the trap device 18 described above, for example trap devices constructed as shown in FIGS. 14 and 15 are known. FIGS. 14 and 15 are cross-sectional views illustrating the construction of typical trap devices.
The trap device shown in FIG. 14 comprises a cylindrical vessel (casing) 22 having apertures at both ends. One aperture of this vessel 22 is employed as a gas inlet port 24 and the other aperture of this vessel 22 is employed as a gas outlet port 26, respectively. Within vessel 22, there is provided a cylindrical baffle plate 28 which is closed at one end. Baffle plate 28 is arranged in the vicinity of the middle of the interior of vessel 22, with its closed end facing gas inlet port 24. Within this baffle plate 28, there is provided a cooling pipe 30 comprising a cooling medium inlet port 30a and cooling medium outlet port 30b. Also, on the wall surface of vessel 22, there is provided a cooling pipe 32 comprising a cooling medium inlet port 32a and cooling medium outlet port 32b. A cooling medium such as water is circulated in these cooling pipes 30 and 32.
Exhaust gas evacuated from the airtight vessel flows from gas inlet port 24 into the interior of vessel 22 and, passing between the inside wall of vessel 22 and baffle plate 28, flows from gas outlet port 26 into the downstream exhaust path. This exhaust gas carries heat. On the other hand, vessel 22 and baffle plate 28 are cooled to a temperature lower than the temperature of the exhaust gas by means of cooling pipes 30 and 32. As a result, the exhaust gas solidifies in the vessel 22, and products generated within the airtight vessel are precipitated as solids. These solids are deposited on the wall surface of vessel 22 and the surface of baffle plate 28.
Also, in the trap device shown in FIG. 15, a cooling pipe 34 comprising a cooling medium inlet port 34a and cooling medium outlet port 34b is provided within vessel 22. This cooling pipe 34 is of a shape that is bent a plurality of times, so the contact area between the exhaust gas and cooling pipe 34 is increased, and the efficiency of collection of the solidification constituents and solids is increased.
Next, a typical example of the construction of the filter 20 referred to above is illustrated in FIG. 16. FIG. 16(A) is a cross-sectional view showing an example of construction of the filter. FIG. 16(B) is a perspective view with part of this filter disassembled.
The filter shown in FIG. 16 comprises a cylindrical vessel 36 having two apertures 38 and 40. The first aperture 38 of this vessel 36 is used as a gas inlet aperture and the second aperture 40 of this vessel 36 is used as a gas outlet aperture, respectively. In this example, the second aperture 40 is formed at one end of vessel 36 while the first aperture 38 is formed in the cylindrical surface nearer to the other end of vessel 36.
A filter mesh 42 is provided in the interior of vessel 36. This filter mesh 42 is constituted by winding a stainless steel plain fabric diamond wire diameter mesh (hereinbelow abbreviated to xe2x80x9cmeshxe2x80x9d) 44 onto the outside of a stainless-steel cylindrical frame 46 (in a condition in which frame 46 is inserted facing in the direction shown by arrow a in FIG. 16(B)). This filter mesh 42 is arranged such that its inside (on the side of frame 46) communicates with second aperture 40 and its outside (on the side of mesh 44) communicates with first aperture 38. A plurality of apertures 46a are formed on the cylindrical surface of frame 46 so that exhaust gas that flows into the first aperture 38 passes through the mesh 44 of filter mesh 42 and reaches the second aperture 40. Solids in the exhaust gas are captured by mesh 44.
Also, first aperture 38 could be used as a gas outlet port and second aperture 40 could be used as a gas inlet port. In this case, the exhaust gas flows into the second aperture 40, and the exhaust gas passes through the mesh 44 of filter mesh 42 before flowing out to the outside from first aperture 38.
However, the conventional exhaust gas filtration devices suffer from the following problems.
1) In the trap devices described above, the solidification constituents or solids in the exhaust gas cannot, be completely removed. In order to remove the solidification constituents or solids, it is necessary to cool the exhaust gas, thereby inevitably bringing the solidification constituents or solids in the exhaust gas into contact with cooling parts of the trap device. It is therefore difficult to remove fine particulate products (solids) that do not flow through the vicinity of the cooling parts. Also, even if they do come into contact, it is difficult for fine particulate products that are flowing past with high speed to be deposited and accumulated.
2) The products described above that are not captured by the trap device are removed by a filter provided downstream of the trap device. However, although this filter is able to remove the fine particulate products due to the fact that it consists of fibrous members of a fine close construction, it is easily blocked even by a very small quantity of particulate products, severely lowering the conductance of the exhaust path. When the conductance has been lowered to a certain degree, it is necessary to wash or change the structural components of the exhaust gas filtration device. Consequently, due to the employment of a filter, the period of continuous use of the exhaust gas filtration device and the semiconductor manufacturing equipment employing is shortened.
3) In order to solve the problem of 2) above, the amount of fine particulate products flowing into the filter must be reduced. Noting that the collection efficiency of the trap device is inversely proportional to the flow velocity of the exhaust gas flowing through the interior, it might be considered that it would be effective to deliberately reduce the conductance at an arbitrary position within the exhaust path. However, if the conductance is lowered to such a level as to solve the problem of 2) above, the load applied to the vacuum pump becomes large, giving rise to the fresh problem of damage to the vacuum pump.
4) Also, in the conventional trap devices, there was local accumulation of solids on the cooling pipes in the vicinity of the gas inlet aperture but scarcely any accumulation of the solids was found on cooling pipes remote from the gas inlet aperture. Consequently, reduction in the conductance of the gas flow path due to accumulation of solids proceeds locally in the vicinity of the inlet port. This therefore shortens the time of use until the trap device must be changed or washed, and means that satisfactory performance in regard to collection efficiency is not achieved.
The object of the invention of this application is therefore to provide an exhaust gas filtration device and auxiliary filtration device aiming at improving the collection efficiency of solidification constituents and solids in exhaust gas, yet in which the period of continuous use can be extended without damaging the vacuum pump.
A further object of this application is to provide a trap device in which the period of use of the device can be extended and the collection efficiency of solids can be improved by promoting deposition of solids at locations other than the vicinity of the gas inlet port.
In order to achieve this object, an exhaust gas filtration device according to the present invention comprising a trap device and a filter arranged successively in an exhaust path of an airtight vessel evacuated by a vacuum pump, for removing solidification constituents and solids in the exhaust gas evacuated into this exhaust path, further comprises: an auxiliary filtration device arranged in the exhaust path between the trap device and the filter.
By thus providing an auxiliary filtration device, some of the solids which were difficult to collect and accumulate by the trap device are removed by this auxiliary filtration device upstream of the filter. Consequently, early blockage of the filter can be prevented, enabling the life of the exhaust gas filtration device as a whole to be extended.
Also, by the provision of an auxiliary filtration device, the conductance of the exhaust path can be deliberately reduced to an extent such that the vacuum pump is not damaged. As a result, the flow velocity of exhaust gas within the trap device is lowered i.e. the dwell time of exhaust gas within the trap device is extended, and the collection efficiency for solidification constituents and solids in the trap device is improved.
Also, according to the invention of the auxiliary filtration device, there are provided a vessel, exhaust gas inlet pipe, exhaust gas outlet pipe and filter element constituting a device for removal of solids in exhaust gas discharged into an exhaust path and arranged in this exhaust path of an airtight vessel evacuated by a vacuum pump.
According to the present invention, this filter element is a sponge-like aggregate constituted by collecting a large number of strip-shaped or filamentous members.
Also, according to the present invention, the interior of the vessel and the exhaust path are connected by an exhaust gas inlet pipe and exhaust gas outlet pipe.
Furthermore, according to the present invention, a filtration region, a first diffusion region and second diffusion region are defined in the interior of the vessel, and the first and second diffusion regions are separated from each other by a filter element arranged in the filtration region.
Furthermore, according to the present invention, the end of the exhaust gas inlet pipe constitutes a gas inlet port and is arranged in either the first or second diffusion region, and the end of the exhaust gas outlet pipe constitutes a gas outlet port and is arranged in either the first or second diffusion region.
Furthermore, according to the present invention, the exhaust gas flow path extending from the gas inlet port to the gas outlet port is constructed such that exhaust gas passes through the filter element at least once.
Usually, this auxiliary filtration device is arranged between the trap device and the filter. The filter element that is installed in this auxiliary filtration device is a member that captures principally comparatively large fine products (solids) that are not removed by the trap device and that cause blockage of the filter.
With such an auxiliary filtration device, the exhaust gas flows into the first or second diffusion region through the exhaust gas inlet pipe. After this, the exhaust gas passes through the filter element and reaches the gas outlet port arranged in the first or second diffusion region. The exhaust gas is then fed to the downstream waste path by the exhaust gas outlet pipe. In this process, solids in the exhaust gas are removed.
Also, thanks to the provision of the filter element, the conductance of the exhaust path at the position where the filter element is arranged is lowered. The flow velocity of the exhaust gas flowing through the exhaust path upstream of the position where the conductance is lowered is therefore reduced. As a result, the efficiency of collection of solidification constituents and solids in the other filtration device arranged upstream of this auxiliary filtration device is improved.
Preferably in the auxiliary filtration device of the present invention, part of the through-flow path of the exhaust gas constitutes a path whereby exhaust gas flows in the opposite direction to the direction in which exhaust gas is evacuated from the airtight vessel.
With such an arrangement, the direction of through-flow of the exhaust gas from the gas inlet port to the gas outlet port is practically opposite to the direction of inflow of exhaust gas into the vessel from the gas inlet port and to the direction of outflow of exhaust gas to the gas outlet port from the interior of the vessel. The flow velocity of the exhaust gas flowing through the interior of the exhaust path upstream of the auxiliary filtration device is thereby further reduced, as a result of which the efficiency of collection of solidification constituents and solids in the other filtration device arranged upstream of this auxiliary filtration device can be expected to be further improved.
Also, in a preferred example of the auxiliary filtration device of the present invention, it is preferable that the gas inlet port is arranged in the first diffusion region and the gas outlet port is arranged in the second diffusion region, the exhaust gas inlet pipe being coupled to the exhaust path through a partition on the side of the second diffusion region of the vessel, while the exhaust gas outlet pipe is coupled to the exhaust path through a partition on the side of the first diffusion region of the vessel.
With this arrangement, a portion of the exhaust gas through-flow path becomes a path whereby the exhaust gas flows in the opposite direction to the direction whereby the exhaust gas is evacuated from the airtight vessel.
Also, in another preferred example of the auxiliary filtration device of the present invention, it is preferable that a partition that divides the first diffusion region into an exhaust gas inlet region and an exhaust gas outlet region, and the filtration region into two, namely, a first and second filtration region, is provided in the vessel; the gas inlet port is arranged in the exhaust gas inlet region and the gas outlet port is arranged in the exhaust gas outlet region; the exhaust gas inlet pipe is coupled with the exhaust path through a partition on the side of the second diffusion region of the vessel; and the exhaust gas outlet pipe is coupled with the exhaust path through a partition on the side of the first diffusion region of the vessel.
With this arrangement, a portion of the exhaust gas through-flow path becomes a path whereby the exhaust gas flows in the opposite direction to the direction whereby the exhaust gas is evacuated from the airtight vessel.
Also, preferably, in the auxiliary filtration device of the present invention, the filter element is constituted by a plurality of metal strips which are packed substantially uniformly between a plurality of support plates having at least one aperture.
Furthermore, in implementation of the auxiliary filtration device of the present invention, preferably a cooling mechanism is provided for cooling the filter element.
Also, it has been noted that generation of solids depends also on pressure, not solely on temperature.
Specifically, in a trap device according to the invention relating to the present embodiment, a trap device arranged on an exhaust path of an airtight vessel evacuated by a vacuum pump, for removing solidified gas as solid in this exhaust path, is constituted by a vessel having in its interior a gas flow path connected to the exhaust path, the flow velocity of gas in the flow path being controlled to a certain flow rate in accordance with position on this flow path.
In this way, the flow velocity of gas is controlled in accordance with flow path position, so accumulation of solids is promoted where the flow velocity is comparatively small. On the other hand, accumulation of solids is avoided where the flow velocity is comparatively large. It is therefore possible to cause solids to be accumulated at prescribed positions on the flow path and to prevent accumulation of solids at locations where lowering of the conductance of the flow path is not desired. Accumulation of solids can thereby be promoted in locations other than the vicinity of the gas inlet port, thereby enabling the period of use of the device to be extended and also improving the efficiency of collection of solids.
In a preferred example of the trap device of the present invention, the flow path comprises a main flow path extending in helical fashion and an auxiliary flow path branched from part of this main flow path and connected to another part of this main flow path.
With such a construction, the gas flowing in the main flow path is slowed down by the gas flowing in from the auxiliary flow path at points where the main flow path and auxiliary flow path merge. The dwell time of the gas in the device is thereby extended and accumulation of solids is promoted. Also, accumulation of solids in the main flow path is promoted as the period of use of the device increases, causing the cross-sectional area of the flow path to be reduced, but, since the gas flows into the downstream part of the main flow path through the auxiliary flow path, the downstream part of the main flow path can also be effectively utilized. Consequently, the period of use of the device can be extended compared with conventional devices.
Also, in a trap device according to the present invention, preferably, the aforementioned main flow path is formed by a thin plate connected to the surface of a shaft element provided in the interior of the vessel, and the auxiliary flow path is formed by an aperture formed at a prescribed position of the thin plate.
Also, in another preferred example of the trap device of the present invention, the flow path comprises a plurality of annular first flow paths and second flow paths connected between the first flow paths, and the flow path cross-sectional area of the first flow paths is changed at prescribed positions.
If such a construction is adopted, the gas flows in each of the first flow paths and second flow paths. The gas flowing into the first flow paths from the second flow paths is branched into two streams. Due to the provision of prescribed locations where the flow path cross-sectional area changes in the first flow paths, the gas proceeds through respective locations where the flow path cross-sectional area is small and where the flow path cross-sectional area is large. In locations where the flow path cross-sectional area is small, the gas flow velocity becomes faster than where the flow path cross-sectional area is larger. Consequently, it is more difficult for solids to accumulate in the locations where the flow path cross-sectional area is small, while, on the other hand, solids accumulate more easily where the flow path cross-sectional area is large. Thus, locations where accumulation of solids is promoted and locations where lowering of the conductance is prevented can be set up at prescribed positions. Consequently, it is possible to induce non-local accumulation of solids, so that the downstream sections of the first flow paths are also effectively used. Consequently, the period of use of the device is longer than conventionally.
Also, preferably, in a trap device according to the present invention, the first flow path is formed by a plurality of thin plates connected to the surface of a shaft element arranged in the interior of the vessel, the second flow path is formed by apertures formed in prescribed positions of the thin plates, and the flow path cross-sectional area of the first flow paths is changed by forming a step at a prescribed position of the thin plates.
Also, preferably, the thin plates are bent in irregular or undulating fashion. This is because the surface area of the thin plates is thereby increased, increasing the effective area on which solids can be accumulated. Also, the gas flow path is extended, enabling the period of use of the device to be extended and the collection efficiency of solids to be improved.
Furthermore, preferably, an irregular structure is formed in the surface of said thin plates. In order to achieve this, for example, an irregular surface may suitably be formed by subjecting the surface of the thin plates to blast processing. As a result, the surface area of the thin plates is increased.
Furthermore, suitably, a cooling mechanism may be provided in the interior of the shaft element referred to above.